....really keen and it was the first instance I saw of the "casualties or suppression" mechanic. -Ivan Sorenson
A great way to re-use your old 40K models. Want to be an Inquisitor with his retinue? Read the Eisenhorn/Ravenor novels and I bet you'll be reaching for the download button. This was created by Craig Cartmell and the Forge of War development group, who also did FUBAR. It comes with warband lists, weapon lists and a campaign (mission) supplement with scenario/opfor generators.
As a small warband-level game using 40K models, this helps to scratch the Necromunda itch.

...an article would be longer than the game but it deserves a nod - Ivan Sorenson

This is a very short (one page) set of rules, which has been re-used in a wide range of genres. The original is modern/near future sci fi but there are a myriad of variants ranging from Star Wars to Stargate, Aliens to medieval, fantasy, horror and VSF. Would work from skirmish to platoon+ level. There are some quite polished unit cards, resources and expansions made by an inventive community. The author Craig Cartmell has gone on to make the commercial VSF rules In her Majesty's Name.

A section/platoon level game. Amongst other stats, units have "motivation" (fanatic/warriors/reluctant/unwilling) that influences their behaviour and would allow a degree of solitaire play. Has an alternating (dice based) activation, with opportunity (reactive) fire. Includes rules for vehicles and dropships. I haven't come across this before, but I'm giving it a +1. Thanks, Ivan!

I'll save Ivan's blushes here and nominate this myself. Alongside FUBAR this is probably the first rule set I'd recommend to someone "getting into" sci fi gaming. One of the most complete free rules sets.
Aimed at the 40K level (platoon+ support/vehicles) this is relatively "hard" near future sci fi/modern with suppression and casualties emphasized (although there are rules for simple psychic powers). Quite polished, it's better laid out than most "paid" PDFs. I remember liking the recon stage (which is sort of a mini-game allowing you to place units before the battle). There are plenty of bells and whistles (army /vehicle builder etc) as well as campaign generators which add flavour - and reveal the author's RPG ancestry. I'd label this the next logical "step up"from FUBAR or similar one-page style rules.

Full Thrust is the best fleet starship game I've ever played. It uses
plotted movement, but the rules are light enough to survive a lot of
tinkering, and the game plays fast enough that you can have a dozen guys
around the table without anyone waiting too long to do something. It
also uses a clever damage tracking system that leaves your half dead
ships clinging to enough life to affect the battle, with the rare
catastrophic BOOM! that lights up the skies. - Warren Abrox

It's a testament to the game (and perhaps a lack of innovation in starship rules) that this continues to be the benchmark, 20 years on. It combines "build you own" rules, and simplicity with easy "mod-ability" and has been adapted for genres from attack helos, to tanks, to VSF.

This game pretty much pioneered the near-future "hard" sci fi genre - many more recent games owe much to its influence.

GZG set the benchmark for not only 15mm, but near future sci fi rules - and is perhaps a reason for the Vietnam-in-space focus of the genre. Stargrunt is their platoon-level rules. Like Dirtside, it isn't easy on the eyes. The rules themselves need a good tidy up. It's based on troop quality & confidence, and you can see the influence it has had on other rules designed since (Tomorrow's War comes to mind.). There are no superheroes - this is about suppression, maneuver and good use of cover.

I don't really game at this scale, but it has a points system and vehicle design system. I found the rules download a bit inaccessible and "heavy going"but it's better designed than it appears. Alternate movement and opportunity fire. Better than Future War Commander commercial rules.

Very quick, very dirty and even for today it can boast with a number of highly original features:

-
Units are either gangs (punx, rebels, scavengers) or squads (troopers,
cops, also rebels) and both units have slighty different rules for
morale and suppression.- No unit coherence necessary if your unit is
equipped with headsets, otherwise to change orders you need to be in
"calling distance, something I remember like 12".- Units can only
act according to their orders which give huge boni on certain actions
and disallow others. (Run, Snipe, Assault, Take Cover etc.)- Some (easy to remember) tables to check for suppression, order acceptance, shooting.-
Shooting is tricky. Base number to hit is 6 on D6 (!) that means a lot
of cinematic dakka dakka. (Unless one side is using snipe orders or
brings in heavy rocket launchers). Once hit however, it's deadly. 2-5d6
added together (2d6 light handgun; 5d6 rocket launcher) and if > or =
model's toughness (usually 7) its dead, if its at least equal to
toughness-5, model is suppressed.

Only problem I remember is that the core rules are gritty cyperpunk and lack anything from vehicles to monsters to psi/magick.

Actually the "chat aloud as you design rules" is something I like - I've actually got a similar post on making a Necromunda/Mordhiem game in my drafts folder, and "Delta Vector the Game" is a reason I started this blog:

" We believe that when a single company controls both the rules and the models for a game system, it inevitably leads to a conflict of interest between what’s best for the players, and what’s best for the company’s own bottom line. "

I never bothered to delve into it, as it seems to be a "Make 40K but better" by 40K-centric players (I'd be interested to know what other games they've played besides 40K) - which is something most of us have done at some stage.

It's something most players go through.

1. Start playing 40K2. Get competitive/join club/get big army3. Start to get disillusioned with balance/price/insert reason4. Create 40K homebrew rules <--_Warstrike is here5. Realise no one wants to play them/others need it to be " official" so they might as well play other, cheaper games6. Switch to other games (Warmachine if competitive, other indie games if not)7. Return to 40K cos of the player base/universe, but on a more restrained scale8. Eventually get sick of GW business practices9. Vow never to ever buy one of their products again

....the conflict is between what’s best for the players (i.e. good, fun, balanced, tactical game), and what’s best for the company’s own bottom line.

Basically, it soon becomes that the main purpose/design goal of the rules is only to sell (and facilitate the sale of) miniatures (which are always the more profitable), rather than be a good, balanced, game in itself.

Witness 40K turning from a characterful semi-RPG skirmish game, to a platoon-company level game to which its mechanics are not particularly suited.

The M42 guys talk a lot about it (with their typical 40K-centric worldview) in I'd tend to agree with a lot of their points (see WHY DON'T WE SELL MINIATURES halfway down)